This invention relates to metal-to-ceramic seals and is concerned more particularly with the sealing of an annular metal member onto a cylindrical ceramic element.
Such seals may be used for example in electrochemical cells of the kind having a solid ceramic electrolyte. Sodium sulphur cells are typical examples of such cells; these have to operate at elevated temperatures where the electrode materials are liquid. The seals necessary to seal these electrode materials within the cell have therefore not only to withstand highly reactive materials at these temperatures but are also subjected to temperature cycling. As a typical example, a sodium-sulphur cell might have a beta alumina tube closed at one end forming the electrolyte with the sodium on the face of the tube, preferably the outer face and the sulphur/sodium polysulphides on the other face. The cell has to be sealed to prevent escape or mixing of these materials and a number of proposals have been made for various types of sealing arrangements for such cells. Compared with most metals, ceramic materials are generally weak, particularly in tensile strength and it is necessary in any seal for such a cell to ensure that the ceramic material is not overstressed.
In a sodium-sulphur cell, the beta-alumina electrolyte tube may have an alpha-alumina tubular extension at its open end so that the end portion is not ionically conductive. The alpha-alumina and beta-alumina have similar coefficients of thermal expansion and it is well known that the alpha-alumina extension tube can be secured to the beta-alumina with a glass seal. In this case, the closure of the cell has to be effected by sealing to the alpha-alumina tube but it is the same problem of forming a metal-to-ceramic seal on the end of a ceramic tube.